Plant for production of aluminum by electrolysis

ABSTRACT

In a plant for production of aluminum by electrolysis comprising a plurality of cells in series, a specified arrangement of the connecting means from cell to cell in order to reduce the horizontal components of the electric current in the cells.

United States Patent 1 Schmidt-Batting Nov. 27, 1973 PLANT FOR PRODUCTION OF ALUMINUM [56] References Cited BY ELECTROLYSIS UNITED STATES PATENTS [75] Inventor: Wolfgang Schmidt-Batting, Chippis, 3,575,827 4/1971 Johnson 204/244 X Switzerland 3,607,685 9/1971 Johnson 204/244 X 3,617,454 11/1971 Johnson [73] Asslgneel SWISS Alummlum, PP 3,616,318 10 1971 Johnson 204 244 x Switzerland [22] Filed: Aug. 25, 1971 Primary Examiner-John H. Mack Assistant Examiner-D. R. Valentine [2]] App! 1742890 Attorney-Robert E. Burns et al.

[30] Foreign Application Priority Data 57 ABSTRACT Sept. 1, 1970 Switzerland 13100/70 In a plant for production of aluminum by electrolysis comprising a plurality of cells in series, a specified ar- [52] US. Cl. 204/244, 204/243 M rangemem of the connecting means from cell to cell in [51] Int. Cl. C22d 3/02, C22d 3/12 order to reduce the horizontal components of the [58] Field of Search 204/243 M, 243 R, electric current in the Cells 1 Claim, 4 Drawing Figures Pmm uuuvzmzs 3,775,281

Y SHEET 3 CF 3 Fig. 4 I

PLANT FOR PRODUCTION OF ALUMINUM BY ELECTROLYSIS .oxide (A1 alumina) is dissolved in a fluoride melt.

Electrolysis is carried out in a temperature range of about 940 to 975C. The cathodically deposited aluminium collects under the fluoride melt on the bottom of the cell. Anodes of amorphous carbon are clipped from above into the melt. The electrolytic decomposition of the alumina causes oxygen to form on the anodes and this combines with the carbon of the anodes to form CO and CO A typical aluminium electrolysis cell is shown diagrammatically in FIGS. 1 and 2 of the accompanying drawings which are a longitudinal section and a transverse section.

A fluoride melt (the electrolyte) is contained in a steel pot 12 in which is a layer of insulation 13 and a carbon lining 11. The insulation 13 is of refractory thermally insulating material. Cathodically precipitated aluminium 14 collects on the bottom 15 of the cell. The surface 16 of the liquid aluminium acts as the cathode. lron cathode bars 17 are embedded in the bottom of the carbon lining 11 and serve to conduct current from the bottom of the cell to the exterior. Anodes 18 of amorphous carbon dip into the fluoride melt from above so as to conduct the direct current to the electrolyte. They are fixed by rods 19 and clamps 20 to two anode bars 21. These together constitute an anode beam. The electrolyte 10 is covered with a crust 22 of solidified melt and on top of this is a layer 23 of alumina. The distance d from the underside 24 of the anode to the upper surface 16 of the aluminium (also called interpolar distance) canbe varied by raising or lowering of the anode beam 21,21 with the aid of the lifting mechanisms 25 which are mounted on columns 26. As a result of attack by the oxygen liberated in the electrolysis, the anodes are consumed on their underside to an extent of about 1.5 to 2 cm each day according to the particular construction of the cell. The cathode bars 1'7 have two tasks. They collect the current from the active part of the carbon bottom beneath the anodes 18 and they conduct it out of the cell. Where they collect the current and are designated by 29, the current intensity in the cathode bar increases on both sides towards the exterior. At 28, outside the active part 27 of the carbon bottom, the cathode bars serve as pure current conductors. From each cell cathode bus bars 30 conduct the current from terminals at the outer ends of the cathode bars 17 to the anode beam 21, 21 of the following cell.

To reduce heat losses here the cross section of the iron cathode bars is reduced outside the active part 27 of the carbon bottom. Thus the flow of heat out of the melt through the bars to the exterior is reduced. This reduction is the subject of our Patent Application No. 139,154 (30.4.71), now U.S. Pat. No. 3,736,244. The electrolyte possesses a substantially poorer electric conductivity than the liquid aluminium which is situated on the bottom of the cell. The ratio of the two conductivities is between 10*: l and 10: 1.1fthe current withdrawal through the carbon bottom does not locally exactly correspond to the current feed through the anodes of the electrolyte, horizontal current density com ponents must occur in the melt, which are caused by the local difference between feed and withdrawal of the current. The great difference of the electrical conductivities of the two stratified liquids has the effect, according to the tangent law of electric current dynamics, that a kink occurs in the current lines at the boundary surface between electrolyte and liquid aluminum. The result is that the current lines in the electrolyte, to a first approximation, extend vertically. On the other hand, in the metal major horizontal current density components can occur which can be locally greater than the vertical. The different current density components in the electrolyte and in the liquid aluminium, in cooperation with the magnetic induction between the two media, result in differences in the pressure which can be compensated only by a doming up of metal. This can amount to many centimetres in height since the domed-up metal is covered by the electrolyte and thus has only an effective specific gravity corresponding to the difference in density between electrolyte and metal.

Furthermore, the horizontal current density components in cooperation with the magnetic induction can cause a force field distribution in the liquid metal which is not free from rotation. The consequence of this is a flow of metal combined with a major doming up of metal which in turn is caused by current density components induced by this movement of a current conductor in a magnet field. Doming up and movement of metal are detrimental to the electrolytic efficiency (ratio of the quantity of aluminium actually obtained to the quantity theoretically precipitated according to Faraday). If the electrolytic efiiciency falls the electric energy consumption rises (kWh/kgAl). If therefore only vertical current density components are present in the metal and in the melt, a doming up of metal without metal movement is impossible. Nevertheless, there may be a rotation drive in the metal, as shown by the following equation of the volume forces k:

Here, j,, j and j signify the current density components in the metal in the three axial directions and 8,, B and B the corresponding components of the magnetic induction. If it is ensured that the current withdrawal through the carbon bottom at the underside of the liquid metal corresponds to the current feed at the upper side of the metal, the following components are zero:

j and j and thus also the three partial derivatives of Only the last member of the rotary drive must be caused to disappear, by making 88 /62: become little or zero, since j; is always present (normal electrolytic current). Normally, horizontal current components occur in both axial directions. In my co-pending Application Ser. No. 174,892 filed Aug. 25, 1971, now U.S. Pat. No. 3,728,243 (corresponding to Swiss Application No. 13101 I describe the simultaneous use of four features in a cell, by means of which the transverse horizontal current density components are largely suppressed and longitudinal components are reduced. No continuous iron current conductors are present in the longitudinal direction of the cell. Nevertheless, substantial horizontal current density components can persist in the cell longitudinal direction in the liquid aluminum unless by suitable dimensioning of the cathode bus bars which conduct the current from the one cell to the anodes of the following cell of the series it is ensured that each cathode bar of the cell bottom as far as possible carries the same current.

The present invention is concerned with achieving this uniformity of current.

According to this invention a plant for the production of aluminium by electroysis of alumina in a melt, comprises a plurality of cells in series, each cell comprising a pot having a carbon bottom in which are embedded a plurality of like parallel horizontal cathode bars which each extend to at least one terminal outside the pot, and an anode beam (the last one to give its current to the cathode bus bar and the last one before the current enters the anode beam of the followng cell points A, B, C, etc., in FIG. 4, carrying anodes arranged to dip into the melt in the pot, and electrical connecting means between each cell and the next in the series, each connecting means comprising a plurality of cathode bus bars each of which connects a respective group of at least one of the cathode bar terminals of one cell to the anode beam of the next cell, the cross sections of the individual bus bars being such that, when an equal current flows through each cathode bar, then the voltage drop is the same along each bus bar from the respective bar terminal nearest to the anode beam to a point midway along the anode beam.

FIG. 4 shows an equivalent resistance substitute circuit diagram calculated from the liquid aluminium of one cell to the middle M of the anode beam of the following cell. R B is the proportional bottom resistance for an iron cathode bar, calculated from the liquid aluminium to the outer end of the cathode bar. A first cathode bus bar collects the current from n cathode bar terminals and has an electric resistance R from the last cathode bar terminal (point A) to the commencement of the anode beam of the following cell. Analogously the second cathode bus bar with its own resistance R from its last cathode bar terminal (point B) to the commencement of the anode beam of the following cell collects the current from n terminals, a third bus bar, with its own resistance R the current from n terminals and so on. R is the resistance of the anode beam of the following cell calculated to the middle M of the anode beam. I is the total cell current.

Each cathode bar should conduct the same current I,,. No horizontal current density components occur in the longitudinal direction of the cell in the liquid aluminium if the cross sections of the individual bus bars are so chosen that the voltage drop in each cathode bus bar, from the point of feed of the last iron cathode bar terminal (points A,B,C etc) to the middle M of the anode beam of the following cell is the same. In this case in the first bus bar a current n I flows, in the second bus bar nzlg, in the third bus bar a current r1 1 and so on. The calculation must take place as if the current I from the anode beam of the following cell were not tapped continuously but at a point exactly in the middle of the cell (point M).

FIG. 3 of the accompanying drawings is a diagrammatic plan of an actual layout. It shows a series of three cells A,B,and C. In this example each cell includes three groups D,E,F of iron cathode'bars on each side. Each group comprises three iron cathode bars G, H, J and a respective bus bar K. In this example two bus bars K are connected to the left end of the anode beam and one bus bar K to the right end. L denotes the direction of the pot line current. A complete series comprises from a few cells up to or more. At the first cell of the series the invention is only to be applied to the bus bar connection to the second cell. At the end of the series, all bus bars are connected together. The number of the iron cathode bars depends on the size of the cell, on the current intensity and on several other factors; for example, a 100,000 Ampere cell can include between 10 and 20 cathode bars (meaning between 10 and 20 protruding ends on each side; often the cathode bars are divided in the middle of the carbon bottom, that is to say that two halves are disposed in such a way that they have a common axis but do not touch each other). As to the number of bus bars, there are many possibilities from one bus bar for each cathode bar to only one bus bar for all cathode bars together on each side.

In FIG. 3 the cells are end to end. They may alternatively be side by side.

The anode beam can consist of one or more single anodic bus bars. In FIG. 2 the anode beam 2L consists of two anodic bus bars.

I claim:

1. A plant for the production of aluminum by electrolysis of alumina in a melt, comprising a plurality of cells in series, each cell comprising a pot having a carbon bottom in which are embedded a plurality of like parallel horizontal cathode bars each having at least one terminal extending outside the pot, and an anode beam carrying anodes arranged to dip into melt in the pot, and electrical connecting means between each cell and the next in the series, each connecting means comprising a plurality of bus bars each of which connects a respective group of at least one of said cathode bar terminals of one cell to the anode beam of the next cell, the cross sections of the individual bus bars being such that, when the equal current flows through each of said cathode bars; then the voltage drop is the same along each bus bar from the respective bar terminal nearest to the anode beam to a point midway along the anode beam. 

1. A plant for the production of aluminum by electrolysis of alumina in a melt, comprising a plurality of cells in series, each cell comprising a pot having a carbon bottom in which are embedded a plurality of like parallel horizontal cathode bars each having at least one terminal extending outside the pot, and an anode beam carrying anodes arranged to dip into melt in the pot, and electrical connecting means between each cell and the next in the series, each connecting means comprising a plurality of bus bars each of which connects a respective group of at least one of said cathode bar terminals of one cell to the anode beam of the next cell, the cross sections of the individual bus bars being such that, when the equal current flows through each of said cathode bars; then the voltage drop is the same along each bus bar from the respective bar terminal nearest to the anode beam to a point midway along the anode beam. 